The present disclosure relates to a stacked type falling film evaporator, a zero liquid discharge system comprising the same, a zero liquid discharging method using the same. In particular, two evaporators may respectively have evaporation tubes with a relatively short length that are installed in a two-stage stacked type evaporator in a vertical direction, thereby addressing the problems of evaporators with reduced efficiency due to formation of a dry zone inside the evaporation tubes, providing easy installation and maintenance due to modularization, simplifying pipelines, and reducing a site area required for installing equipment.
The stacked type falling film evaporator may be applied to a zero liquid discharge system for desulfurized wastewater in power plants or industrial wastewater, but is not restricted to the above and may be applied to various industry fields where a high-degree evaporation process, seawater desalination, or salt production processes are useful.
Recently, companies and academic circles are becoming more and more interested in Zero Liquid Discharge (ZLD) systems. There are indications of a gradual increase of the supply price of industrial water, a gradual increase of production costs due to an increase of discharge fees by regulations on total quantity of effluent water, and a scheduled enactment of a recycling obligation of more than ⅓ of effluent discharge flow. Also, companies and academic circles value judgments are changing to avoid concerns over environmental and civil concerns. Recently, there has been a movement to introduce zero liquid discharge systems not only for specific wastewater but also for all water.
It is estimated that approximately 100 zero liquid discharge systems are now operational in Japan, and thousands of zero liquid discharge systems are operational in the United States.
The zero liquid discharge systems in Japan are mainly installed in high valued added semiconductor factories but require a great deal of high quality water and have significant installation and operating expenses because the installation regions are limited to specific contaminant discharge areas, such as national parks.
For example, the Canon company is located in Oita, Japan and manufactures cartridges for photocopiers. Oita is incorporated in a limited immutable weight region and a ban was requested on wastewater discharge by the nearby National Federation of Fisheries Cooperatives. Hence, the Canon company introduced a zero liquid discharge system. As another example, UMC Japan, which is a semiconductor manufacturing factory, introduced a zero liquid discharge system because the factory was located in a national park.
Zero liquid discharge systems in the United States have been introduced mainly in areas where strict effluent water quality standards were applied, when effluent water quality standards were established by the state, or in factories located in regions lacking an abundant supply of water such as deserts.
As an example, La Paloma Plant is a thermoelectric power plant located in the middle of the Mojave Desert in California. It introduced a zero liquid discharge system because very strict effluent water quality standards were applied. The plant is located in a large-scale agricultural area, water supply from nearby areas of the Mojave Desert is not good, and the price of water is very high. La Paloma Plant reuses recovered water as boiler makeup water for running a power plant turbine. As another example, Intel, which is a semiconductor factory, also introduced a zero liquid discharge system because of strict effluent water quality standards and industrial water shortages in the climate of Arizona.
This kind of ZLD, which is a process of reusing treated sewage water and of discharging a small quantity of sludge excluding the treated water, is divided into two types. A Reverse Osmosis (RO) ZLD is a separation process using reverse osmosis. A thermal ZLD evaporatively concentrates and phase changes by heating. Evaporative concentration technology utilized in the 19th century food industry and intensified environmental regulations leading to increased reuse of water resources have increased demands in thermal ZLD technology applied in various industry fields.
The thermal ZLD process using phase change be heating is mostly effective in non-degradable wastewater. A waste heat steam heating type uses a boiler, an evaporation type operates in a vacuum decompressed state, and a flame direct heating type uses methods of heating wastewater.
The waste heat steam heating type requires a large-scale infrastructure, a large installation scale and excessive installation costs. A flame direct heating type provides a treatment effect like a simple evaporation type but has increased energy consumption.
The vacuum decompressed evaporation type provides an energy savings because it is a method of evaporation by lowering the boiling point of wastewater while keeping the pressure inside the evaporator in a vacuum condition. But this type has several disadvantages including that the treatment result may be non-uniform dependent on conditions of the introduced wastewater, and that it is difficult to maintain and repair due to maintenance of the vacuum condition.
A schematic basic structure of a falling film evaporator is illustrated in FIG. 1.
The evaporator 1 includes: a cylindrical housing 10; a flow uniformity device 20 provided by a plate horizontally mounted at an upper portion of the housing 10; and a wastewater inlet 11 mounted higher than the flow uniformity device 20, such that introduced wastewater is supplied to an upstream space S1 of the upper portion of the flow uniformity device 20.
A plurality of evaporation tubes 30 are densely mounted inside the housing 10. Upper ends of the evaporation tubes 30 penetrate the flow uniformity device 20 in such a way that the upstream space S1 communicates with an inner space of the evaporation tubes 30. Therefore, wastewater supplied into the upstream space S1 flows down along the inner walls of the evaporation tubes 30. The wastewater evaporated and concentrated while passing through the evaporator 1 is collected in a liquid storage tank S3 of the evaporator 1. The wastewater flowing down along the inner walls of the evaporation tubes 30 forms a falling film that is evaporated while being heated by heat exchange with vapor disposed on the outside of the walls of the evaporation tubes 30. The vapor is introduced into a heat exchange space S2 in the middle of the housing 10 via a vapor inlet 40. A vapor outlet 50 extracts vapor having undergone heat exchange in the heat exchange space S2 of the housing 10 and discharges the extracted vapor from the evaporator 1.
Such a falling film evaporator may restrain a rise of the boiling point without pressure loss inside the device and reduce contact time with a heating body, such as vapor, because a heat transfer rate is very high even though there is a difference in temperature with a heating fluid. Moreover, the falling film evaporator may reduce temperature increase of liquid that is sensitive to heat because a thermal gradient in a liquid film kept at about 1 mm to 2 mm is very small.
Furthermore, the falling film evaporator may sufficiently transfer heat even though a temperature difference between the heating fluid and the inside fluid becomes less than 10° C. because the heat transfer rate is very high. The inside fluid temperature may be maintained and not rise excessively.
The falling film evaporator has further advantages in that a period of time required for achieving steady-state operating conditions of the evaporator is relatively short and a period of time required for stopping operation is also short because the volume of the wastewater inside the tube is small. It is effective in low enriched fluid evaporation and power consumption of a circulation pump may be kept low.
However, if the wastewater film is broken, scale may be formed on the inner walls of the tubes. The falling film evaporator is difficult to employ when the wastewater includes lots of scale components. It also may require a distributor design technology to distribute wastewater to each of the tubes uniformly, for example, to prevent the film from breaking.
In more detail, the falling film evaporator includes long tubes of more than 10 m in length. Therefore, the falling film evaporator has several disadvantages in that an evaporative concentration efficiency of the entire device is reduced because a zone where the wastewater film is dried, namely a dry zone, is highly likely to be formed in the proximity of the downstream side, in that workability is deteriorated at the time of installation or maintenance, and in that it takes a relatively long time to achieve operational conditions because the tubes are long.